Generally, electromechanical transducer is a device that converts mechanical movement into an electrical signal or vice versa. Some of the widely used electromechanical transducers are piezoelectric sensors or piezoelectric actuators. Description of piezoelectric actuators is given below.
Piezoelectric actuator is a device that generates mechanical movements according to electrical signals. There have been various efforts to use piezoelectric actuator as a substitute for other actuators such as hydraulic actuators, electric motors used in the field of mechanics, aeronautic and space engineering or civil structures, etc.
However, conventional monolithic piezoelectric actuators generate very small deformation in response to induced piezoelectric actuation with longitudinal deformation rate of 0.1%-0.2%, and are too brittle and liable to fracture when subjected to stress due to its bad fatigue characteristics. There have been much effort to solve this problem, and many kinds of piezoelectric actuators with various amplification mechanisms have been developed.
Among the amplifying actuators, actuators that amplify displacement by using bending of the material or longitudinal expansion of piezoelectric material without any additional external device are called Internally Leveraged or Bender actuators. These include unimorph or bimorph actuators, Reduced And INternally Biased Oxide Wafer (called ‘RAINBOW’) and Thin layer UNimorph DrivER (called ‘THUNDER’), etc.
Above mentioned unimorph actuator consists of piezoelectric layer attached on metal layer (51) as shown in FIG. 1, and bimorph actuator consists of two piezoelectric layers attached to each other. These actuators are actuated by the tension or contraction generated by the electric field applied to the piezoelectric layer which in turn bend the entire actuator. Although these unimorph and bimorph actuators generate relatively large actuating displacements, the actuating force is quite small.
The RAINBOW actuator is produced by reducing one surface of the piezoelectric material containing lead such as PZT (composite oxide of lead, zirconium and titanium) through chemical reaction. To describe the process in more detail, the piezoelectric material is heated on a block of graphite at a high temperature of 975° C. so that oxygen is emitted from the piezoelectric material at the boundary surface of graphite and piezoelectric material, thereby producing a single structure of combined piezoelectric and non-piezoelectric metallic layer. When this structure is cooled at a room temperature, the structure has the form of dome with curvature due to the difference of the heat expansion coefficients of two layers. This actuator can generate large displacement through pumping motion.
The THUNDER actuator is developed by Langley laboratory of NASA and is produced by attaching thin piezoelectric ceramics and metal plate by using high temperature hardening adhesive. THUNDER is produced at hardening temperature of 320° C. in an Autoclave. THUNDER, like RAINBOW, is given a curvature due to the difference of heat expansion coefficients of the ceramics and metal, and gets a large displacement due to a pumping movement generated by the electric potential applied on the piezoelectric layer. These types of actuators with a curvature show improved force characteristics compared to the unimorph or bimorph actuators due to the enhanced self stiffness in the normal direction of the plane.
Meanwhile, LIPCA (LIghtweight Piezo-composite Curved Actuator), an improved form of THUNDER, has been developed by substituting metallic layer of THUNDER with fiber-reinforced composite material. LIPCA is hardened at 177° C. as in the process of manufacturing fiber-reinforced composite material, and has a simpler manufacturing process than THUNDER since there is no need of using adhesive due to the adhesive property of the fiber-reinforced prepreg. LIPCA is also lighter than THUNDER by about 40%, and has the merit of producing various forms of actuators by using the change of structural performance according to the lamination of the composite materials.
FIG. 2 shows displacement-force relationship of piezoelectric actuators currently on the market, and it can be seen that THUNDER has outstanding displacement characteristics. Although THUNDER is known to have the best performance among the bender-type actuators developed so far, it has the disadvantage of requiring special adhesive for using at high temperature and the poling process at room temperature due to the loss of piezoelectric characteristics of piezoelectric ceramics at high temperature. Also, LIPCA, which has simpler manufacturing process compared to THUNDER, requires auto-clave equipment during composite manufacturing process and long time for hardening. Thus, these actuators have disadvantage of requiring long manufacturing time.